A&A 599, A67 (2017) Astronomy DOI: 10.1051/0004-6361/201629733 & c ESO 2017 Astrophysics A test of Gaia Data Release 1 parallaxes: implications for the local distance scale Stefano Casertano1; 2, Adam G. Riess2; 1, Beatrice Bucciarelli3, and Mario G. Lattanzi3 1 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: [email protected] 2 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA 3 INAF, Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, TO, Italy Received 16 September 2016 / Accepted 23 November 2016 ABSTRACT Aims. We present a comparison of Gaia Data Release 1 (DR1) parallaxes with photometric parallaxes for a sample of 212 Galactic Cepheids at a median distance of 2 kpc, and explore their implications on the distance scale and the local value of the Hubble constant H0. Methods. The Cepheid distances are estimated from a recent calibration of the near-infrared period-luminosity (P–L) relation. The comparison is carried out in parallax space, where the DR1 parallax errors, with a median value of half the median parallax, are expected to be well-behaved. Results. With the exception of one outlier, the DR1 parallaxes are in very good global agreement with the predictions from a well- established P–L relation, with a possible indication that the published errors may be conservatively overestimated by about 20%. This confirms that the quality of DR1 parallaxes for the Cepheids in our sample is well within their stated errors. We find that the parallaxes of 9 Cepheids brighter than G = 6 may be systematically underestimated. If interpreted as an independent calibration of the Cepheid luminosities and assumed to be otherwise free of systematic uncertainties, DR1 parallaxes are in very good agreement (within 0.3%) with the current estimate of the local Hubble constant, and in conflict at the level of 2.5σ (3.5σ if the errors are scaled) with the value inferred from Planck cosmic microwave background data used in conjunction with ΛCDM. We also test for a zeropoint error in Gaia parallaxes and find none to a precision of ∼20 µas. We caution however that with this early release, the complete systematic properties of the measurements may not be fully understood at the statistical level of the Cepheid sample mean, a level an order of magnitude below the individual uncertainties. The early results from DR1 demonstrate again the enormous impact that the full mission will likely have on fundamental questions in astrophysics and cosmology. Key words. parallaxes – distance scale – cosmology: observations – stars: variables: Cepheids – space vehicles: instruments 1. Introduction Hipparcos (Perryman et al. 1997; van Leeuwen 2007) and Tycho-2 (Høg et al. 2000) catalogs, parallaxes and proper mo- The Gaia mission (Prusti 2012; Gaia Collaboration 2016b), tions could be determined as part of the Tycho-Gaia astrometric launched in December 2013 by the European Space Agency, solution (TGAS; Michalik et al. 2015) and are included in DR1. will revolutionize our knowledge of individual stellar objects These stars are typically brighter than visual magnitude 11.5 and and of the structure of the Milky Way (MW) by providing dis- have typical parallax uncertainties of 300 µas. tance and velocity measurements of unprecedented precision for One of the early difficulties encountered by the Gaia mis- over a billion individual objects within the MW. Gaia end-of- sion has been a somewhat unexpected variation of the Basic mission results will also have a major impact on cosmology, Angle, the angular separation between the two fields of view, enabling the determination of the Hubble constant H0 to better with an amplitude of several mas, as reported by the on-board than 1%, assuming concomitant control of statistical and system- Basic Angle Monitor (BAM). Data taken during commission- atic errors beyond the parallax measurements (Riess et al. 2016, ing (Mora et al. 2014) prove that the variation reported by the hereafter R16). The mission is expected to last at least 5 years, BAM is real. Self-calibration procedures have been devised to and the final astrometric measurements, with expected accuracy correct for the Basic Angle variation, and it is expected that the ∼ 5−10 µas for the best-measured stars, will be released in 2022. residuals are degenerate with a global zero point error in the par- The Gaia DR1 (Gaia Collaboration 2016a), which took place allax (Michalik & Lindegren 2016). A sample of 135 000 AGN September 14, 2016, is based on measurements taken within has been used as part of the solution and its verification; from the first 14 months of observations (Lindegren et al. 2016), an analysis of parallaxes for both Hipparcos stars and AGN, and includes data for over a billion stars brighter than mag- Lindegren et al.(2016) suggest that parallax zero point errors nitude 20.7. Due to the short observing period and the lim- are .0:1 mas in magnitude, with possible zonal variations and ited number of separate measurements for each target, par- color terms, and a systematic difference between northern and allax and proper motion are partially degenerate, and for a southern hemispheres. majority of the stars only positions are available in DR1. Lindegren et al.(2016) o ffer several observational and sci- However, for about 2 million stars in common with the entific tests and validations of the TGAS results. Because of Article published by EDP Sciences A67, page 1 of6 A&A 599, A67 (2017) the inherent challenge in reaching a new level of parallax pre- by R16. In order to reduce the impact of object-by-object redden- cision, it is important to produce additional tests of the set of ing and extinction, the P–L relation is often formulated in terms Gaia parallax measurements. Here we employ an additional of a so-called Wesenheit magnitude (Madore 1982; Macri et al. validation, based on the sample of 249 Galactic Cepheids in 2015, see R16 for details of our implementation). Wesenheit van Leeuwen et al.(2007), at a median distance of 2 kpc. A sim- magnitudes are formed by subtracting from the primary mag- ilar test, based on the sample of Cepheids in Fouqué et al.(2007) nitude a color term in the same direction as the reddening law; if but excluding nearby Cepheids (within 1 kpc), is reported in the spectra are smooth and the reddening law is well constrained, Sect. C.3 of Lindegren et al.(2016). Our analysis is comparable a Wesenheit magnitude is then reddening-free, in the sense that to theirs, and leads to similar results. However, we use a more re- a given source will have the same Wesenheit magnitude when cent Cepheid calibration (R16) with well-understood uncertain- exposed to different degrees of reddening. For Cepheids, We- ties, and we are able to obtain a quantitative assessment of the senheit magnitudes, especially those for which the primary filter reported DR1 errors, and to derive interesting, albeit tentative, is in the NIR, have the additional advantages that the color term implications on H0. We also find some evidence for a modest is small and insensitive to the reddening law, the P–L relation is parallax underestimate for the brightest and nearest Cepheids, insensitive to metallicity, and its intrinsic width is reduced. We which are explicitly excluded from the Lindegren et al.(2016) use the same quantity as in R16: analysis. W The Cepheid P–L relation, called the Leavitt Law mH = m160 − 0:3861(m555 − m814) (1) (Leavitt & Pickering 1912), provides a tight correlation between period and luminosity of Fundamental Mode Cepheids and has where m160, m555, and m814 are the Vega-system magnitudes been central to the determination of the scale of the Universe in the WFC3 filters F160W (NIR), F555W, and F814W, with for a century. Yet most Galactic Cepheids are at a distance central wavelength 1537, 531, and 802 nm, respectively. The of a few kpc; until recently, this placed them well beyond the coefficient 0:3861 is appropriate to a Galactic reddening law useful range for measurements of accurate trigonometric paral- (Fitzpatrick 1999) with R = 3:3. Since the available photom- laxes. Benedict et al.(2002, 2007) used the Fine Guidance Sen- etry in van Leeuwen et al.(2007) is ground-based, while the sors (FGS) on the Hubble Space Telescope (HST) to measure best calibration of the P–L relation obtained in R16 is based the parallaxes of ten Cepheids within 0:5 kpc, reaching indi- on HST data, we use the ground-to-HST transformations given vidual precisions of 150 to 300 µas, or an average of 8% per in R16, Eqs. (10)−(12). The systematic uncertainty in this trans- object. More recently, Riess et al.(2014) and Casertano et al. formation, found by R16 to be 0.013 mag, is small compared to (2016) measured two Cepheid parallaxes using HST Wide Field the 0.05 mag precision for the mean of the sample (see Sect. 3). Camera 3 in scanning mode, with precision of 54 and 38 µas, However, to retain the full precision of future Gaia Cepheid respectively. With the addition of a few Hipparcos measure- parallax measurements, it will be crucial to measure the MW ments with errors of 300 µas, the resulting sample of 15 Galactic Cepheid mean magnitudes directly with HST, as discussed in Cepheids had a weighted mean precision of 2.1% and provided Sect. 5. one of three anchors for the determination of the Hubble con- We adopt the primary calibration of the P–L relation from stant (R16; see also Riess et al. 2011; Freedman et al. 2012). As R16: discussed in Sect.